How to extrapolate data from data

There are many occasions when a column of data needs to be created from an already existing column for ease of data manipulation. For example, perhaps you have a body of text as a pathology report and you want to extract all the reports where the diagnosis is ‘dysplasia’.

You could just subset the data using grepl so that you only get the reports that mention this word…but what if the data needs to be cleaned prior to subsetting like excluding reports where the diagnosis is normal but the phrase ‘No evidence of dysplasia’ is present. Or perhaps there are other manipulations needed prior to subsetting.

This is where data accordionisation is useful. This simply means the creation of data from (usually) a column into another column in the same dataframe.

The neatest way to do this is with the mutate function from the {dplyr} package which is devoted to data cleaning. There are also other ways which I will demonstrate at the end.

The input data here will be an endoscopy data set:

Age <- sample(1:100, 130, replace = TRUE)
Dx <- sample(c("NDBE", "LGD", "HGD", "IMC"), 130, replace = TRUE)
TimeOfEndoscopy <- sample(1:60, 130, replace = TRUE)

library(dplyr)

EMRdf <- data.frame(Age, Dx, TimeOfEndoscopy, stringsAsFactors = F)

Perhaps you need to calculate the number of hours spent doing each endoscopy rather than the number of minutes

EMRdftbb <- EMRdf %>% mutate(TimeOfEndoscopy / 60)

# install.packages("knitr")
library(knitr)
library(kableExtra)

# Just show the top 20 results

kable(head(EMRdftbb, 20))

Age	Dx	TimeOfEndoscopy	TimeOfEndoscopy/60
69	HGD	9	0.1500000
51	IMC	36	0.6000000
55	IMC	32	0.5333333
91	HGD	13	0.2166667
6	NDBE	59	0.9833333
69	HGD	9	0.1500000
44	HGD	5	0.0833333
98	LGD	32	0.5333333
66	LGD	34	0.5666667
58	LGD	35	0.5833333
48	IMC	17	0.2833333
56	IMC	30	0.5000000
16	IMC	33	0.5500000
73	LGD	13	0.2166667
23	IMC	17	0.2833333
75	HGD	60	1.0000000
19	IMC	16	0.2666667
70	NDBE	8	0.1333333
21	IMC	17	0.2833333
91	NDBE	30	0.5000000

That is useful but what if you want to classify the amount of time spent doing each endoscopy as follows: <0.4 hours is too little time and >0.4 hours is too long.

Using ifelse() with mutate for conditional accordionisation.

For this we would use ifelse(). However this can be combined with mutate() so that the result gets put in another column as follows

EMRdf2 <- EMRdf %>%
  mutate(TimeInHours = TimeOfEndoscopy / 60) %>%
  mutate(TimeClassification = ifelse(TimeInHours > 0.4, "Too Long", "Too Short"))

# Just show the top 20 results

kable(head(EMRdf2, 20))

Age	Dx	TimeOfEndoscopy	TimeInHours	TimeClassification
69	HGD	9	0.1500000	Too Short
51	IMC	36	0.6000000	Too Long
55	IMC	32	0.5333333	Too Long
91	HGD	13	0.2166667	Too Short
6	NDBE	59	0.9833333	Too Long
69	HGD	9	0.1500000	Too Short
44	HGD	5	0.0833333	Too Short
98	LGD	32	0.5333333	Too Long
66	LGD	34	0.5666667	Too Long
58	LGD	35	0.5833333	Too Long
48	IMC	17	0.2833333	Too Short
56	IMC	30	0.5000000	Too Long
16	IMC	33	0.5500000	Too Long
73	LGD	13	0.2166667	Too Short
23	IMC	17	0.2833333	Too Short
75	HGD	60	1.0000000	Too Long
19	IMC	16	0.2666667	Too Short
70	NDBE	8	0.1333333	Too Short
21	IMC	17	0.2833333	Too Short
91	NDBE	30	0.5000000	Too Long

Note how we can chain the mutate() function together.

Using multiple ifelse()

What if we want to get more complex and put several classifiers in? We just use more ifelse’s:

EMRdf2 <- EMRdf %>%
  mutate(TimeInHours = TimeOfEndoscopy / 60) %>%
  mutate(TimeClassification = ifelse(TimeInHours > 0.8, "Too Long", ifelse(TimeInHours < 0.5, "Too Short", ifelse(TimeInHours >= 0.5 & TimeInHours <= 0.8, "Just Right", "N"))))

# Just show the top 20 results

kable(head(EMRdf2, 20))

Age	Dx	TimeOfEndoscopy	TimeInHours	TimeClassification
69	HGD	9	0.1500000	Too Short
51	IMC	36	0.6000000	Just Right
55	IMC	32	0.5333333	Just Right
91	HGD	13	0.2166667	Too Short
6	NDBE	59	0.9833333	Too Long
69	HGD	9	0.1500000	Too Short
44	HGD	5	0.0833333	Too Short
98	LGD	32	0.5333333	Just Right
66	LGD	34	0.5666667	Just Right
58	LGD	35	0.5833333	Just Right
48	IMC	17	0.2833333	Too Short
56	IMC	30	0.5000000	Just Right
16	IMC	33	0.5500000	Just Right
73	LGD	13	0.2166667	Too Short
23	IMC	17	0.2833333	Too Short
75	HGD	60	1.0000000	Too Long
19	IMC	16	0.2666667	Too Short
70	NDBE	8	0.1333333	Too Short
21	IMC	17	0.2833333	Too Short
91	NDBE	30	0.5000000	Just Right

Using multiple ifelse() with grepl() or string_extract

Of course we need to extract information from text as well as numeric data. We can do this using grepl() or string_extract() from the library(stringr).

Let’s say we want to extract all the samples that had IMC. We don’t want to subset the data, just extract IMC into a column that says IMC and the rest say ’Non-IMC’

Using the dataset above:

library(stringr)

EMRdf$MyIMC_Column <- str_extract(EMRdf$Dx, "IMC")

# to fill the NA's we would do:EMRdf$MyIMC_Column<-ifelse(grepl("IMC",EMRdf$Dx),"IMC","NoIMC")

# Another way to do this (really should be for more complex examples when you want to extract the entire contents of the cell that has the match)

EMRdf$MyIMC_Column <- ifelse(grepl("IMC", EMRdf$Dx), str_extract(EMRdf$Dx, "IMC"), "NoIMC")

So data can be usefully created from data for further analysis.

Hopefully this way of extrapolating data and especially using conditional expressions to categorise data according to some rules is a helpful way to get more out of your data.

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This article originally appeared on https://sebastiz.github.io/gastrodatascience/ and has been edited to render in Quarto and had NHS-R styles applied.